Validity of Freehand 3-D Ultrasound for Skeletal Muscle Volume Assessment: A Systematic Review and Meta-Analysis

Skeletal muscle volume is a fundamental component of overall health. Magnetic resonance imaging (MRI) is a reference standard for quantifying muscle volume, but it has limited accessibility. Freehand 3-D ultrasound, which combines B-mode imaging and 3-D tracking technology, is an accessible alternative for muscle volume assessment; however, its validity in measuring muscle volume across diverse populations and muscle groups is not well understood. A systematic literature search was conducted in PubMed, Web of Science, Scopus and Google Scholar until August 2025. Studies reporting Pearson's correlations, mean differences and limits of agreement for muscle volume between freehand 3-D ultrasound and reference standards in vivo in humans were included. Random-effects models were used to pool effect sizes using multi-level analyses. Mean differences and limits of agreement are presented as a percentage of muscle volume. Ten studies met the inclusion criteria, which evaluated limb and trunk muscles from healthy and clinical populations. All reference measures of muscle volume were from MRI. Meta-analysis revealed very strong correlations between freehand 3-D ultrasound and MRI (r = 0.99, 95% confidence interval [CI]: 0.97-1.00, p < 0.001). There was no systematic difference between freehand 3-D ultrasound and MRI muscle volume (-2.55%, 95% CI: -6.63 to 1.52%, p = 0.219), which displayed relatively narrow limits of agreement of ∼10% (95% CI: 6.82-14.37%, p < 0.001). Freehand 3-D ultrasound represents a valid approach to quantify skeletal muscle volume in vivo across diverse populations and muscle groups; however, its ability to detect changes over time requires validation.